1. Field of the Invention
The present invention generally relates to control of vehicles which may be operated in both hovering and flying operational modes or regimes and a transitional mode between them and, more particularly, to providing optimal unified guidance control in the transition region as well as in the hovering and flying modes.
2. Description of the Prior Art
Many types of vehicles are known for transporting equipment and/or personnel to desired locations in various environments for which they are, generally, specially adapted. One broad category of such vehicles is adapted to operate through and immersed in a fluid medium such as air or water including undersea vehicles and so-called lighter-than-air aircraft or where the weight of the vehicle is balanced by lift from surfaces which move relative to the vehicle such that movement of the vehicle is not required to produce such lift and as in helicopters, so-called ground effect vehicles and the like. Accordingly, such vehicles are capable of operating in both a flying mode or regime and a hovering mode or regime.
In a flying regime, such a vehicle utilizes control surfaces to generate forces against the surrounding fluid medium and thus point the vehicle along a desired path while the vehicle is being propelled in a given direction, generally axially of the vehicle. The efficacy of control surfaces generally diminishes with diminishing relative speed of the vehicle relative to the fluid medium but can be reasonably efficient to fairly low speeds. In contrast, in a hovering regime where the vehicle is substantially stationary, maneuvering is generally accomplished by thrusters which develop forces to move the vehicle in a desired direction or orient it to a desired attitude which may be substantially arbitrary with reference to the vehicle (e.g. along or around any of three orthogonal axes—only motion around two such axes corresponding to pitch and yaw resulting in or being analogous to pointing in a flying mode although roll may be used in conjunction therewith). The efficacy and efficiency of thrusters to provide guidance of the vehicle diminishes rapidly with increasing speed while, in sharp contrast with deflection of control surfaces in a relatively flowing medium, substantial energy is required to provide thrust forces. While their respective modes and principles of operation and relative effectiveness under various conditions are very different, the ultimate function of both control surfaces and thrusters is to develop forces and moments to alter the position and/or attitude of a vehicle and thus control surfaces and thrusters are collectively referred to as effectors.
Control of control surfaces and control of thrusters to cause a desired change in position or attitude of a vehicle are fundamentally different and, in the past, have been separately controlled through separate systems in response to separate operator inputs. Therefore, there has been a significant and frequently encountered possibility of seeking to control the vehicle through the wrong mode; the correct mode often being ambiguous due to, for example, variable currents in the fluid medium imparting virtual velocity to the vehicle when the vehicle position is substantially stationary or vice-versa. That is, a “flying” vehicle relies on forces developed by control surfaces moving through a fluid medium to alter vehicle attitude and thus uses attitude commands to effect guidance while a hovering vehicle relies upon thrusters to control both vehicle attitude and translation rate but uses translation rate to effect vehicle guidance. Control of the vehicle in either of these regimes is straightforward but becomes complex when the vehicle must have the capability to both hover and fly, particularly where motion of the fluid medium may be irregular and/or unpredictable. These complexities are compounded by the fact that effectiveness of both control modalities are oppositely affected and in different degrees by vehicle speed through the fluid medium and the fact that use of thrusters is much more expensive in terms of energy use than use of control surfaces.
More specifically, the traditional approach to this problem has been to design a set of control laws or rules appropriate to the flying regime and another set of laws or rules appropriate to the hovering regime and then to provide a strategy or method to handle the transition from one regime to the other. In general, a preferred strategy has been to simply provide switching from on control regime to the other at a particular threshold speed which is generally set as low as possible consistent with effectiveness of control surfaces at the threshold speed in view of the much greater energy requirements for use of thrusters. However, such an approach becomes problematic where the threshold speed is comparable to the variation in flow rate of the fluid medium; causing excessive switching or use of a control regime which is actually ineffective to provide the desired control. Setting a higher threshold speed is not a practical solution to such a problem in view of the disparity of energy requirements between control surfaces and thrusters and the greater use of thrusters implied by the higher threshold speed.